In positive-displacement compressors, capacity control may be obtained by both speed modulation and suction throttling to reduce the volume of vapor or gas drawn into a compressor. Capacity control for a compressor can provide continuous modulation from 100% capacity to less than 10% capacity, good part-load efficiency, unloaded starting, and unchanged reliability. In some positive-displacement compressors, capacity can also be controlled by a slide valve employed within the compressor. The slide valve can be operated to remove a portion of the vapor from the compression chamber of the compressor, thereby controlling the capacity of the compressor. Besides the slide valve, other mechanical devices, such as slot valves and lift valves, may be employed in positive-displacement compressors to control capacity. Adjustments to capacity control valves or variable displacement mechanisms can meet the demands of the system. In a refrigeration system, capacity can be regulated based upon a temperature setpoint for the space being cooled. In other systems in which the compressor is processing gas, capacity may be regulated to fully load the torque generator or prime mover (turbine or engine drive) for the compressor. However, all of the currently available methods are expensive and add to the initial cost of investment in the equipment.
In chiller applications where economy is desired both in the initial cost of the system and in operation of the system, a variable volume ratio application is desired. The volume, or compression ratio Vr in a screw compressor, is the ratio of the volume of a groove at the start of compression to the volume of the same groove when the discharge port begins to open. Hence, the size and shape of the discharge port is a factor in determining the volume ratio of a screw compressor.
For maximum efficiency, the pressure generated within the grooves during compression should exactly equal the pressure in the discharge line when the volume begins to open to it. If this is not the case, either overcompression or undercompression occurs, both resulting in internal losses. Furthermore, overcompression can harm the compressor. Such losses increase power consumption and noise, while reducing efficiency. Thus, volume ratio selection desirably should be made according to operating conditions when such an adjustment is available.
If the operating conditions of the system seldom change, it is possible to specify a fixed-volume ratio compressor that will provide good efficiency. Because overcompression can damage a compressor, when designing such a compressor, it is designed so that it does not frequently operate in an overcompression mode, if at all. As a result, such a compressor is designed to run at maximum compression under the most severe operating conditions, meaning that such a compressor runs in undercompression modes at all other operating conditions, so that inefficiency may result over extended periods of operation. What is needed is a system that permits adjustments to the volume ratio that changes the volume ratio depending on the conditions that the compressor experiences. This will allow the compressor volume to be adjusted to change the volume, and hence the volume ratio, as operating conditions change, allowing the compressor to operate at maximum efficiency.